1. Field of the invention
The present invention relates to an electric rotating machine in which a rotor is provided with a permanent magnet that supplies magnetic flux to an armature core in cooperation with a field winding.
2. Description of the Related Art
In charging generators for use in vehicles, for example, it is a recent trend that higher power, downsizing and weight saving have been increasingly demanded in order to improve current consumer and fuel economy required by the vehicles. To meet this demand, an attempt has been proposed to reduce leakage flux between magnetic poles, increase effective magnetic flux and improve power output by adding magnetizing force with a permanent magnet to magnetic field of a generator, as disclosed in the Japanese Patent Publication No. 2865091.
In the generator of such arrangement, however, even if the power is generated only with the permanent magnet without using field current, power generation level comes to exceed an electric load when the generator is rotated at high speed under the small electric load. As a result, there is a disadvantage of breakage of battery due to overcharge or application of abnormal voltage to the electric load.
To overcome this disadvantage, one of prior arts employs an arrangement consisting of at least two types of rotors, that is, a first rotor provided with a field winding and a second rotor provided with a permanent magnet, and switching means for changing a value of field current flowing through the mentioned field winding and changing a direction of flow, as disclosed in the Japanese Patent Publication No. 3063106. In such a prior art, when the electric load is large, the current value and flowing direction are controlled by the switching means so that a predetermined magnetic flux is generated with respect to the field winding in the same direction as the permanent magnet. On the other hand, when the load is small and magnetic flux from the permanent magnet is sufficient for the power output, the current value and flowing direction are controlled by the switching means so that the magnetic flux generated by the permanent magnet is cancelled by the magnetic flux generated in the field winding. Thus, the output voltage is adjusted to be a suitable value, resulting in prevention from over-voltage.
In the technology disclosed by the above-mentioned Japanese Patent Publication No. 3063106, however, a problem exists in that it is necessary to be provided with at least two types of rotors of a first rotor provided with a field winding and a second rotor provided with a permanent magnet, the arrangement becomes complicated as a whole, which results in increase of weight and cost.
Another problem exists in that it is necessary to separately add the switching means for changing a value of field current flowing through the mentioned field winding and changing a direction of flow depending on the magnitude of electric load, which also results in increase of cost.
Moreover, a further problem exists in that when the rotor is driven at high speed and electric load is small, it is necessary to continuously cause current to flow in a direction of reducing the magnetic flux generated by the permanent magnet for the purpose of suppressing generation of over-voltage, eventually resulting in increase of power consumption.